Various apparatus and methods have been designed to perform therapy and/or diagnostics in the vascular system. In the area of intravascular therapy, heated balloon dilation catheters have been used to deliver heat to the vessel wall. Some of these catheters heat the inflation fluid within the balloon by means of a heating element such as a coil wrapped around the catheter body under the balloon region. The power delivered to the heating element may be controlled by a conventional analog feedback circuit which has one or more temperature sensing devices such as a thermocouple or thermistor secured to the inner surface of the balloon or to the heating element.
Several disadvantages may be associated with such catheters. Because independent sensors are used to detect the temperature of either the heating elements, inflation fluid or balloon surface inaccuracies may result due to defects in the sensor or a limited range of accuracy. In addition, the position of the sensor may lead to inaccuracies. For example, a sensor placed on the balloon surface does not necessarily measure the hottest point on the catheter. This arises because the balloon surface is not heated directly and thus its temperature lags behind the temperature of the inflation fluid. Once the sensor detects that the temperature at a given point on the balloon surface has reached a maximum permissible value, the temperature of other points on the balloon surface may have increased beyond that value due to the higher temperature of the inflation fluid within the balloon. In addition, the temperature of the surface of the balloon is related to the thermal diffusivity of the adjacent tissues. Thus, the temperature at various points along the balloon surface may be different. The sensor detects the temperature at a given point on the surface while the temperature at other points may be higher. This may cause the temperature of the balloon surface to reach a level which could damage the vessel in which the catheter is placed. In addition, the low profile of the catheter may be compromised by the inclusion of a sensor or sensors on the catheter.
Other catheters directly heat the balloon surface itself. For example, U.S. Pat. No. 5,035,694 (Kasprzyk et al.) discloses a catheter having a conductive layer 52 formed on the inside of the balloon. (See FIG. 4). The layer is heated by making contact with conductors 50 and 51 so that the exterior working surface temperature of the balloon is raised. The power supplied to the conductors may be controlled in response to the temperature of the balloon by a suitable feedback control system. (See FIG. 1). The control circuit includes an ohmmeter 34 used to monitor the resistive load of the balloon and control the output of the power source in response thereto. The output of the ohmmeter is compared with a signal representing a desired set point in a controller 35 and fed back to control the power source.
Several disadvantages may be associated with such a catheter. Stresses may be placed on the conductive layer as the balloon is inflated and deflated. This may cause the conductive layer no crack or tear. Also, the properties of the conductive layer may change over a period of time and use thereby leading to results which may not be stable and repeatable over a period of time.
In the area of intravascular diagnostics, various apparatus and methods have been used to measure the flow of blood through a vessel. For example, U.S. Pat. No. 4,920,967 (Cottonaro et al.) discloses a wire guide provided with a Doppler mechanism for determining the blood flow velocity in the region of the distal end of the wire guide. A pulsed Doppler angioplasty guidewire is available commercially from Cardiometrics of Mountain View, Calif. under the trade name FLOWIRE.RTM..
Several disadvantages may be associated with such Doppler guidewires. Using Doppler technology allows such guidewires to measure the peak-to-peak velocity and not necessarily a true mean velocity. (S. Denardo et al., "Advantages of Peak Velocity Over Mean Velocity Measurements Made by Doppler Catheters," Circulation, Vol. 86, No. 4, Supplement I, I-870, October 1992). In addition, such guidewires measure the component of velocity along the sensor axis and thus the measurements made using such systems are dependent upon the angle of the sensor with reference to the vessel.
Other devices not limited to the field of intravascular diagnostics utilize a heated wire to determine characteristics such as the velocity of a flowing fluid. This is commonly referred to as "hot-wire" anemometry. A wire having a resistance proportional to its temperature is heated to a select temperature above the ambient temperature of the fluid in which it is placed. As the cooler fluid flows past the wire, variations are caused in the temperature of the wire and thus its resistance. The resistance of the wire is used to control its temperature through the use of a bridge circuit in which the wire forms one arm of the bridge. As the resistance of the wire changes the balance of the bridge is upset. The temperature of the wire and thus its resistance is increased by increasing the current delivered to the bridge to restore balance. The change in current delivered to the bridge circuit is measured and reflects the velocity of the flowing fluid. Examples of such systems can be found in U.S. Pat. Nos. 3,352,154 (Djorup); 3,438,253 (Kuether et al.) and 5,094,246 (Rusz et al.)
Several disadvantages may be associated with such anemometers that utilize bridge circuits as described above. To provide accurate results, precision power resistors may be required. In addition, in order to vary the temperature at which the wire is to be maintained, a precision power potentiometer may also be required. Use of such power resistors and potentiometers increase the cost associated with such anemometers. In addition, a large power supply may be required because equal current has to be supplied to both arms of the bridge. Using a bridge circuit to provide control thus results in a bulky system with limited accuracy.
It is desirable to provide an apparatus and method for performing intravascular therapy and diagnostics which does not suffer from the disadvantages listed above.